Dollars and Sense

Following is a simple comparison of four chillers: one premium high-efficiency air-cooled screw chiller and three water-cooled centrifugal chillers. Both the air-cooled and water-cooled chillers were selected based on computer modeling, with the software identifying units contributing to optimal life-cycle values. These values were derived from unit efficiency and user-defined first cost, making the selected units the most attractive to own.

Each chiller required a full-load capacity of 385 tons, its performance based on standard Air-Conditioning, Heating, and Refrigeration Institute (AHRI) conditions. Kilowatt-per-ton values were calculated by software and applied to the formula 0.01A + 0.42B + 0.45C + 0.12D (typical of AHRI part-load methodology) to derive chiller-only electricity consumption. An allowance of 0.08 kW per ton (full load, 0.01A) and 0.06 kW per ton (part load, all other points) then was added to each water-cooled chiller to account for the additional cooling-tower fan and pump energy required. The tower fans and condenser pumps were assumed to run at 50-percent speed and 100-percent flow, respectively, during part-load operation. Energy cost was based on 8 cents per kilowatt-hour, typical of the utility rate structure in Houston. Table 2 illustrates the overall energy costs for each chiller. The results show how the models would perform under assigned operational conditions.

The air-cooled chiller had the highest chiller-only operational cost, while the costs associated with each water-cooled chiller dropped as efficiency improved.